however whenever you fill in a cell you also fill in any cells that are set by filling in that cell ie you complete as much of the puzzle as you can given the current state of the grid. You get one point for every cell you fill in.

How do both players determine and agree whether these additional cells are actually forced?Especially when the stronger player applies more advanced technics?

How do we avoid arguments about more advanced technics here?

Note that for example in chess, if the stronger player spots a mate in 10 and the other player doesn't there's no problem -the other player will discover the mate at some point by the most direct constraints of the game!

I think the best way around this problem is to say that if it's possible to fill in a cell without causing an immediate contradiction then you are not allowed to fill in a cell in a way that causes a contradiction.

How do both players determine and agree about contradiction?

if you play this game with two different coloured pens, say a red one for placing the first clue and a blue one for filling in the consequences, then at the end of the game, if you complete the grid, then the red clues will be a valid sudoku puzzle.

Just for a good understanding of what you mean: this part is not necessary for the game except when youwant the valid puizzle as a by-product.

Then when the first player places his clue the next player is restricted to placing his clue in a cell that is symetrically equivalent to that. This way to play first has a slight advantage. However you can give up that advantage by playing in the central square.

ab wrote:I'm wondering if the player playing first has too much of an advantage? It depends how easy it is to take the lead from him. A rule change that would get around this is as follows:

First lets create some terminology. If you place the first number in a group of symmetric cells (whatever the symmetry you're using) we'll say you're starting an orbit. At the moment only one player can start an orbit and he seems to have a big advantage. For instance I could play in r3c4 knowing that evert couldn't play in r2c2 or r2c3 because he had to play in my orbit. An alternative rule could be that each player can start one orbit. Then on any turn you can play in either of the two orbits, but you can only start a new orbit when the orbit you started has been completed. That way you can still sometimes have control over where your opponent plays, but the power does not lie solely with one player.

Ah that answers my last question.Another option might be to allow all symmetries in one game - that gives some freedom...